1. Field of the Invention
The present invention relates to power driven rotary tools such as screwdrivers and drills, and more particularly to such tools having a torque adjusting device which is adapted to adjust the torque for driving a tool bit for rotation.
2. Description of the Prior Art
In general, power driven rotary tools, such as screwdrivers, have a torque adjusting device wherein optimum tightening torque may be selected at will to suit the size of the screw and the nature of the work so as to prevent improper tightening such as insufficient tightening and excessive tightening.
Such a torque adjusting device is disclosed, for example, in U.S. Pat. Nos. 3,001,430, 3,442,360 and 4,265,320. The disclosed device includes a clutch interposed between an electric motor and a tool bit, a coil spring for biasing the clutch in one direction, and a ring-shaped adjusting member adapted to act on the spring. The adjusting member threadably engages the housing of the tool so that when the operator rotates adjusting member, it increases or decreases the spring force to be applied to the clutch. Thus, the device includes an adjusting member threadably engaging the housing and hence, it has the disadvantage that the operator must rotate the adjusting member through a large angle, resulting in a poor efficiency of torque adjusting operation. Further, there is the disadvantage that the device is complicated in construction and cumbersome to use.
In some designs, a power driven rotary tool may include a control member located on the underside of the tool body. For example, Japanese Laid-Open Utility Model Publication No. 59-143670 discloses a screwdriver including a torque adjusting lever for adjusting the biasing force of a coil spring which bears against a clutch, and U.S. Pat. No. 4,161,242 discloses a driver drill having an adjusting lever for shifting the tool between driver mode and drill mode. However, because of the control member provided on the underside of the tool body, these cited tools have the disadvantage that the adjusting operation is cumbersome. Further, there is the disadvantage that the torque setting or the shifting between driver mode and drill mode can not be verified during operation, resulting in reduced ease of operation.